Why Plants Die
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Scale insects on houseplants: bumps on stems and leaves that feed on sap

Check your plant with the questions

Scale insects rarely look like insects at all. What most people notice first is a scatter of small brown, tan or whitish bumps along a stem or beside the central vein of a leaf, or a glossy, tacky film on the leaves below them. Each bump is a waxy shell with a living insect under it or inside it, fixed in place and drinking sap through mouthparts shaped like a fine straw. A handful does a plant little harm. Left alone indoors, where the common species keep breeding through the winter, they can build up until growth slows, leaves yellow and shed, and whole shoots die back.

Is it scale?

Turn the leaves over first. The kinds that live on houseplants tend to settle on the undersides, lined up near the midrib and larger veins, and on young stems. Adult covers run from barely visible specks to about a quarter of an inch long, and they may be round, oval or long and narrow. None of them shows a head, legs or wings you could pick out without a lens. To tell whether one is alive, press it hard with a fingernail: a live scale gives up a bead of fluid, and one that yields nothing is probably dead. Dead scales can stay stuck to a plant long after the problem is over.

Other signs point the same way. Leaves that turn sticky, or develop a black, sooty coat on their upper faces, often lie directly under a colony. Ants climbing a houseplant are worth following, since they come for the sugary waste that many scales produce.

Soft scale or armored scale

The two families found most often behave differently, and the difference changes what will work.

Soft scales are the larger, domed ones, smooth or slightly waxy, reaching about a quarter of an inch. The shell is the insect's own body wall, so flipping one over lifts the whole animal off the leaf. They feed on the sugary phloem sap and pass honeydew in quantity: the sticky excess that drips onto leaves and whatever sits below and lets sooty mold grow. Brown soft scale is one familiar species.

Armored scales are smaller and flatter, under one-eighth inch across, and often carry faint rings around a raised dot of a different color. Their cover is a separate shield: lift it off and the soft body stays behind on the plant. They make no honeydew, so an armored infestation shows up as discolored or dying leaves and twigs rather than stickiness. Small whitish armored kinds turn up on dracaenas and palms, and other armored species on orchids.

Crawlers: the stage that spreads

Most female scales lay their eggs out of sight beneath their own bodies. A few, the cushion scales among them, push the eggs out under a pad of white waxy threads, which can be mistaken for mealybugs or woolly aphids. The young that hatch are tiny yellow-orange crawlers, and they are the stage built for walking. Inside a couple of days they choose a spot, push their mouthparts in and stop moving. Armored scale nymphs lose their legs at that point; soft scale nymphs keep barely visible ones and can shift slowly.

Crawlers also reach other plants on air currents or carried by people. Outdoors many scales manage one generation a year, but those living in greenhouses and homes can breed without a break, so eggs, crawlers and adults may all be present at once.

What they are mistaken for

  • Mealybugs hide under fluffy white wax and cluster in leaf joints and crevices rather than sitting flat along a vein.
  • Whitefly nymphs are flat, pale ovals under the leaves that look very much like young scale. Adult whiteflies flying up from the same plant give them away.
  • Aphids look like ordinary small insects and form colonies around buds and soft new growth.

Getting it under control

Start by moving the plant away from the others, and check its neighbors, since crawlers will have had time to travel. Then work through these, in rough order of effort:

  • Remove what you can see. Scrape the adults and any egg masses off stems and leaves. It helps on a lightly affected plant, though on its own it seldom clears a heavy one.
  • Cut out the worst. A stem that is crusted with scale on a small plant is often better pruned away than cleaned.
  • Spray the young. Horticultural oil, insecticidal soap and neem oil kill by contact and last only a short time, so they need repeating. They act mainly on crawlers and settled nymphs, not on mature adults under their covers. Wet every infested part, including the undersides of leaves. Armored scales are harder to reach than soft ones, so thorough coverage matters more with them. Read the label, since some plants react badly to oil.
  • Consider a nematode for soft scale. The biological control Steinernema feltiae is sold for use against soft scales.
  • Be wary of systemic products. Imidacloprid, a popular systemic, works on soft scales but not on most armored ones. Systemics stay active for months and kill beneficial insects too.
  • Let a plant go. When scale is thick across a whole plant, starting again with a new plant may take less effort than cleaning every stem.

Inspect new plants before they join a collection and, where you can, hold them away from your other plants at first. Adult females barely travel, so scale usually arrives on a plant rather than finding its own way in.

Judging whether it worked

Because dead scales stay attached, the old shells tell you little. Watch the new growth instead: if fresh leaves and stems come through clean, the colony has stopped breeding. Scales that are puffy, darkened or pierced by a tiny round hole have been parasitized by small wasps and will not recover. A loop of double-sided tape wrapped around a stem traps crawlers, which show up on it as tiny yellow-orange dots: a simple way to see whether a new generation is on the move.

Plants whose sources name scale insects

The list for this page is built from the site's own data: it holds each plant whose own entry, at the RHS or at NC State Extension, counts scale insects among its pests.

A plant with sticky leaves and nothing visible on it may have aphids or whitefly instead. If the cause is still unclear, run the diagnosis.

Answer a few questions about your plant

Sources

This page is written for this site from the sources above; it quotes none of them. It has not seen your plant.